Hydrogen is an important element in the replacement of hydrocarbon and carbon based electric power production by sustainable and environmentally appropriate alternative means, for example in hydrogen powered fuel-cells. Hydrogen for this purpose may be produced by utilizing variable energy sources such as atmospheric or photo-voltaic power sources in order to produce and store hydrogen for conversion into power when a demand exists. This removes one of the main objections to installation of alternative renewable power sources, namely that power is often produced at inappropriate times and does not have the availability to conform diurnal power demand variation. Therefore, there has been a large effort in developing technologies to facilitate hydrogen based energy production and storage.
An important element in this hydrogen based energy concept are electrochemical devices that can convert chemical energy stored in hydrogen into electrical energy (hydrogen fuel cells) and, vice versa, convert electric energy into chemical energy for storage by producing hydrogen from water by electrolysis (water electrolyzers).
Solid electrolyte based fuel cells, have matured considerably over the years with respect to providing commercially viable designs and production methods. One aspect of the development has been directed to the design of fuel cell stacks with improved bipolar separator plates functioning as anode/cathode current collectors and as flow plates for handling both cathode side and anode side fluid flow. Such bipolar flow plates may be produced at commercially viable costs, e.g. from carbon/graphite based compression mouldable compounds. For example, EP 1 726 060 B1 discloses a dual function bipolar separator plate for use in a solid polymer electrolyte based fuel cell stack. The bipolar separator plates has on an anterior face an anode flow field, and on a posterior face a cathode flow field. The bipolar separator plate can facilitate transport of reactants and heat to and from the reactive surfaces in order to maintain the electrolytic conversion process and to exhaust the reaction products away. As further discussed in this document, in the context of solid electrolyte technology, separating elements for fuel cells are typically manufactured from conducting carbon composites.
However as mentioned above, a hydrogen based concept of energy production and storage also requires suitable electrolysers for converting electrical energy into chemical energy by producing hydrogen for storage. The hydrogen is stored in gas reservoirs at high pressures, e.g. for later use as a fuel in hydrogen fuel cells. Electrolyser systems therefore comprise means for compressing the hydrogen produced by the electrolyser. In a most preferred configuration, a so-called high pressure electrolyser is adapted to directly produce the hydrogen at high pressures, thus allowing to transferring the hydrogen from the electrolyser exhaust directly to a storage recipient without the need of external compression devices. Thereby the overall efficiency of the energy conversion system is improved.
While the cost for the production of fuel cell stacks have been successfully reduced by the maturing technologies, the same technologies cannot be applied in a straight forward manner to electrolysis devices with solid polymer electrolytes. On the contrary, renewed focus on electrolytic devices for the production of hydrogen from water by electrolysis has revealed numerous challenges for materials used in such electrolysis devices. For example, the anode side environment in a water electrolyser comprises a mixture of oxygen in water, which, under operational conditions of the cell with an applied electrical potential, is highly corrosive for most materials. In particular, the above-mentioned carbon/graphite based flow plates commonly used in PEM fuel cells are not at all suited for this highly corrosive anodic fluid environment. Existing fuel stack designs can therefore usually not be merely operated in a reverse mode in order to achieve water electrolysis. In one approach the carbon/graphite based materials are replaced by corrosion resistant materials, such as titanium. However, this solution is very expensive and not commercially viable on a large scale beyond highly specialised niche applications. Furthermore, shaping of titanium to provide a highly complex three-dimensional patterned flow plate is an expensive and time-consuming task. This adds to the cost and directly contradicts the reduction in cost required for producing a commercially viable device.
U.S. Pat. No. 4,214,969 discloses a bipolar separator plate for a stacked cell water electrolyser. The bipolar separator plate is made of a carbon/graphite based compound and has open-faced channels for the distribution and collection of fluids/gases on both sides of the separator plate. The anode side surface of the bipolar separator plate is sealed by a protective metallic foil. The foil is glued or otherwise applied conform to the surface profile including protruding portions, channel side walls, and the channel bottom. An adequate flow of water through the electrolytic cell may require a certain cross-sectional area of the fluid channels. This could be achieved by widening the channels. However, wide channels of this type would provide insufficient mechanical support for the MEA, in particular in the presence of an elevated cathode side pressure as compared to the anode side pressure. Furthermore, a conform application of a thin metallic foil with thicknesses of about 25 μm on a flow field pattern with a deep and narrow channel profile without puncturing the foil can be a tedious and thus costly task. Furthermore, the disclosed flow field provides an unsatisfactory distribution of water supply over the surface of the MEA resulting in hot spots and/or dry spots affecting the efficiency and lifetime of the electrolytic cells.
Therefore there is a need for an improved electrolytic device for the production of hydrogen from water by electrolysis, which is preferably adapted to be operated at high pressures, which can be operated reliably over a long period of time, and which can be produced at a commercially viable cost.
Object of the present invention is to overcome the above-mentioned disadvantages of known flow plates, or at least provide an alternative.
The object is achieved by a flow plate according to independent claim 1, wherein preferred embodiments are defined by the dependent claims as discussed in the following.
Throughout the application, the term “lateral” refers to directions parallel to a principal plane of a planar element, here of the flow plate, and the term “vertical” refers to directions perpendicular to the plane of the planar element, here of the flow plate. The term “fluid” refers to both gases and liquids or mixtures thereof. The acronym PEM stands for Polymer Electrolyte Membrane, and the acronym MEA stands for Membrane Electrode Assembly.